For some time now high pressure treatment has been used as a method for inactivating micro-organisms and certain enzymes in substances such as foodstuffs and other provisions. A decisive factor for obtaining a good result from a high pressure treatment is that the pressure is sufficiently high. During treatment of food or provisions, the pressure is usually set at a pressure between 1,000–10,000 bar. Such high pressures make however extreme mechanical demands on presses used for high pressure treatment.
It is economically desirable to process food and similar substances with a high throughput. It is known to make high pressure presses with a substantially cylindrical vessel in which a first substance is separated from a second substance, a pressurisation medium, by a floating piston. Pressure exerted by the pressurisation medium is transferred to the substance by the floating piston. This approach offers an advantage of a rapid throughput of product to be treated.
In EP 0 772 981 A1 a high pressure press of the floating piston type is disclosed. The press is made of a cylinder, sealed at the top 3 and bottom, with the enclosed space divided into two chambers by a freely moving piston. One chamber contains pressurisation fluid, while the other contains product for treatment. Each chamber has inlet and outlet pipes equipped with valves. The valves through which product enters and leaves the treatment chamber are incorporated in the top of the press, and the valve which allows pressurisation fluid to be released is incorporated in the base.
However, there are certain disadvantages with known types of free moving or floating pistons with a piston sealing means such as the one described. Under extremely high pressure, a sealing means of the type such as 4A in piston 4 shown in FIGS. 1, 3 of EP 0 772 981 A1 described could be liable to expand under pressure to an extent that does not exactly follow the expansion behaviour of the cylinder. This may be expected with conventional seals of the Bridgeman type under extremely high pressure. A reduced sealing effect of the sealing means between the piston and the cylinder it is in would allow leakage of product and/or pressurisation fluid past the sealing means. This means that either a high pressure press has to be run with longer cycle times at lower pressures to avoid leakage at high pressure, or run at high cycle times with a risk of contamination taking place. A degree of contamination of product is permissible when the pressurising fluid is clean and compatible with the product. However, this imposes further limits in respect of service life for pressurisation fluid and further treatment stages and equipment needed for filtering or cleaning the pressurisation fluid.